"Mark Twain" was also the pseudonym chosen by American humorist Samuel L. Clemens. Supposedly, he chose the name because of its suggestive meaning, since it was a riverman's term for water that was just barely safe for navigation. One implication of this "barely safe water" meaning was, as his character Huck Finn would later remark, "Mr. Mark Twain - he told the truth, mostly." Another implication was that "barely safe water" usually made people nervous, or at least uncomfortable. (Quoted from Answers.com)
In the days before electronic depth sounders, the established means of knowing the depth of water under a boat was a lead line: a lead weight tied on the end of a piece of (often) knotted or otherwise marked rope. This primitive but foolproof device was used from the misty depths of time until this century when sonar was invented and used for locating the bottom.
Also, comes here the origin of the measurement term "fathom." Stretch out your arms to the sides, and most folks will find that the span is about 6 feet: a fathom. So, if you were taking depths with an unmarked lead line, you could still get a pretty accurate measurement using fathoms, measuring the line as you hauled it back in. (But in order to avoid repeatedly hauling in and measuring the wet line by stretching it out with one's arms, it was traditional to tie marks at intervals along the line. These marks were made of leather, calico and other materials, so shaped and attached that it was possible to "read" them on sight by day or at night by the feel of each one.) Nautical charts today still mark depths in fathoms.
When anchoring, it is important to know the depth of the water. Today the electronic depth sounder makes that part easy. But it is equally as important to know how much anchor rode you have put out - it does no good to anchor in an accurately measured depth of 25 feet with 24 feet of rode...
Not surprisingly, similar problems engender similar solutions. We have seen many anchor rode marking solutions: pieces of small line tied into the links of the anchor chain, colored nylon wire ties, and others. But by far the most common means we see on G-dock at Shilshole is carefully applied colored paint.
The usual paint is spray paint, but unlike lead line markings of yore, the length indicating color schemes appear to have no standard - they are unique to each boat.
It is not uncommon to find someone with their bow pulled right up to the dock, and all their anchor chain neatly flaked onto the dock in (presumably) measured lengths.
No matter what scheme is used to mark the rode, it is worn off by going over the anchor windlass wildcat, so it needs to be re-done every few years.